The present invention relates to a control system for a gas distribution system. More particularly, the present invention relates to a control system for maintaining gas distribution system pressures at the remote ends or "low points" of the system.
One of the problems faced by gas distribution companies is the maintenance of minimum service pressures, particularly at points remote from the supply or pressure-regulating station, or the so-called low points of the system, where experience or tests indicate that the lowest pressures in the system will normally exist. Such maintenance of minimum pressures, at all points in the system, is essential from the standpoint of adequate service to the customer as well as safety.
An obvious solution to this problem would be to feed the gas from the supply or pumping station to the distribution system at a rate substantially higher than that required to meet expected consumption and thus maintain the minimum pressures required at all points in the system, including the low points. However, under these circumstances, excessive pressures would occur at points in the system where the pressure is normally high and create its own problems. Safety is, of course, a major problem of excess pressure, but another perplexing and costly problem is excessive leakage losses in the distribution system. As the pressure increases, losses due to leakage in the distribution system increase. Thus, the system pressure should also be kept as low as possible while still maintaining an adequate minimum pressure.
The problem of maintaining minimum and maximum pressures in a distribution system is further complicated by radical variations in gas consumption, due to seasonal demand, as well as unforeseen demand, or unexpected variations in the timing or duration of the peak seasonal loads.
Accordingly, it was common practice for an operator at the supply station to increase the pressure at the supply station, usually in accordance with a schedule based on previous records of consumption during peak seasonal periods, to take care of predictable increases in demand. However, it was still necessary to maintain excess pressure at the supply station during low consumption periods in order to maintain safe minimum pressures at the low points in the event of unforeseen or unexpected contingencies. This, of course, was costly but it was considered justified as insurance against pressures below the required low level.
As systems became larger, semi-automatic control was then instituted. One such system involved the use of a differential booster system at the main station with a telemetered signal to a satellite station in another part of the town plant. This system leaves a lot to be desired, since the control is not based on gas pressures at the low point of the system. Where larger systems supply larger towns or areas, two or more measuring stations are established in the area served. Gas pressures telemetered to these measuring stations are monitored by dispatcher personnel on 24-hour duty, and gas pressures are manually controlled. Utilizing either of these systems, a minimum pressure of 20 lbs. was usually maintained at the low point, which dropped to about 10 lbs. or less during peak demand.
As systems became still larger, and to free dispatchers from performing manual control of the gas, systems were the installed to automatically control the gas pressure in accordance with the measured low pressures. However, such control systems encounter cycling problems which often overboosted the system. Again, a low-point pressure of 20 lbs. was as low as the system could be operated to safely handle demand on peak heating days. In addition, this system did not alleviate the problem of substantial volumes of unaccounted-for gas, due to leakage and other factors.
Present day distribution systems are often a complex network of piping with a given distribution area fed by one or more regulators. Since the system low point is fed by one or more regulators, the regulator setting must be changed periodically to maintain the desired pressure at this system low point. Increase in the system load between the regulator and the low point will cause the low point pressure to drop, requiring the regulator setting to be increased in order to maintain adequate pressure.
To meet these requirements, systems are now in existence in which the low point pressure is continuously measured and telemetered to a dispatch or control center. The telemetered data is displayed on a pressure indicator or recording chart, usually including high and low pressure alarms. The district pressure regulator is, in turn, remotely controlled by a signal telemetered from the control station. When the measured pressure varies from either the maximum or minimum limits, as indicated by the display and alarm, the operator simply pushes a raise or lower button at the control center to raise or lower the pressure. The push button switch, at the control center, telemeters an appropriate signal to a remote set controller or remote set regulator controlling the district regulator, which, in turn, controls an appropriate valve. After "commanding" the regulator, the operator waits a suitable period of time to see if the pressure comes back within limits. If the pressure does not come back within limits, the button is again pushed to send another command, followed by another waiting period. This procedure is repeated as often as necessary until the proper pressure is attained. This system is not without problems since the operators are kept quite busy during peak load periods, which unfortunately is also the time at which the gas take has to be examined very carefully to insure that the distribution companies do not exceed their limits set by the contract with the supplier.
Consequently, automation of the above system is desirable. However, automation by conventional means is difficult because of the time lag between a change in the district regulator setting and the sensing of the change at the low point transmitter. Further, in many cases, each of a plurality of regulators, supplying a single system with a single low point, must be preset, i.e., one regulator at 5 lbs., another at 6 lbs., etc., to initially balance the system. These problems have, however, been overcome by a system in which the sensed, excessively low or high, pressures operate a timer. The timer produces a pulse, of predetermined, adjustable time duration, which is sent to all of the regulators controlling gas to the sensing low point. This is followed by an adjustable waiting period, sufficient to permit stabilization of the change. This "command" and "wait" cycle is repeated (a pulse of the same duration followed by a waiting period of the same duration), until the pressure is within limits, and is automatically discontinued when this point is reached. The pulses are either negative or positive depending upon whether the pressure is to be raised or lowered. The system also is provided with a manual (raise and lower) push buttom command for each regulator, in the event of failure of the automatic system.
As well as this system has served its purpose, it is not without problems. First of all, the manual override is designed to individually control each of a plurality of regulators separately, thus contributing to unbalance in a system which was originally balanced and requiring the same hectic dispatcher operation of the non-automated system. Secondly, no means is provided at the low point telemeter transmitter in the event of failure of the primary unit. Finally, the pressure range between the high and low points is still broader than desirable, thus making the high pressure higher than desirable and contributing to leakage losses between the regulator and the ultimate customer.
It is therefore, an object of the present invention to provide an improved gas distribution system which overcomes the above-mentioned disadvantages. Another object of the present invention is to provide an improved gas distribution system which can be manually operated to simultaneously adjust a plurality of regulators. A further object of the present invention is to provide am improved gas distribution system which automatically adjusts a plurality of regulators in the event of failure of the primary system. Yet another object of the present invention is to provide an improved gas distribution system which can maintain gas pressure within a small range of pressures despite large fluctuations in demand. These and other objects of the present invention will be apparent from the following description.